In general, future-generation communication systems are under development to provide data at high rates to mobile stations, ensuring mobility for the mobile stations. They are also able to provide services with a variety of Quality of Service (QoS) requirements to the mobile stations.
Among the services, real-time services such as Voice over Internet Protocol (VoIP) or gaming service are sensitive to service delay and thus have large limitations.
Especially when a mobile station in a handover zone receives a voice packet service or a gaming service from a base station, it may not receive service data because it is in a poor channel status. In this case, the mobile station requests retransmission of the service data to the base station and the base station should retransmit the service data.
As described above, when the mobile station is in the handover zone, there is a high probability that errors occur during data transmission/reception, resulting in data loss. To reduce the information loss, error control schemes are used, thus increasing system reliability. A major error control scheme is Hybrid Automatic Repeat reQuest (HARQ).
With reference to FIG. 1, the configuration of a typical communication system will be described.
FIG. 1 illustrates the configuration of a typical communication system.
Referring to FIG. 1, the communication system is Universal Mobile Telecommunication System (UMTS). It includes a core network (CN) 100, a plurality of radio network subsystems 110 and 120, and a user equipment (UE) 130. The radio network subsystem (RNS) 110 and RNS 120 include radio network controllers 111 and 112 and a plurality of Node Bs 105, 107, 109 and 111. For example, the RNS 110 has the radio network controller (RNC) 111 and the Node Bs 113 and 115, and the RNS 130 has the RNC 112 and the Node Bs 114 and 116.
The CN 100 provides connectivity between networks. The RNC 111 and RNC 112 each control at least one Node B and the Node B communicates with the UE 130 on a radio channel.
With reference to FIG. 2, a description will be made of a data transmission and reception operation when a user equipment (UE) enters a handover zone during data transmission to, and reception from, a serving Node B in the communication system.
FIG. 2 is a diagram illustrating a signal flow for a data transmission and reception operation when a UE enters a handover zone during data transmission to, and reception from, a serving Node B in the communication system.
Referring to FIG. 2, a radio network controller (RNC) generates data in the form of packet data units (PDUs) and assigns a sequence number to each PDU. It is assumed that the data is real-time data such as those of a gaming service or a voice service. The RNC transmits a PDU with a sequence number k, PDU #k to the serving Node B in step 201. The serving Node B transmits PDU #k and resource information to the UE in step 203. The resource information is information about the packet transmission scheme or the transport format of the data, or allocation information about the position and size of physical resources that carry the data.
In step 205, the UE transmits an ACKnowledgment (ACK) message to the serving Node B, notifying successful reception of PDU #k.
The RNC then transmits a PDU with a sequence number k+1, PDU #k+1, to the serving Node B in step 207. The serving Node B transmits PDU #k+1 and resource information to the UE.
On the assumption that the UE has moved to a handover zone that is under a poor channel status and thus that has neighbor cell interference, the UE fails to receive PDU #k+1 in step 209. In step 211, the UE transmits a Negative ACK (NACK) message to the serving Node B, indicating the reception failure of PDU #k+1.
Meanwhile, if the UE determines that a handover to a target Node B is required, taking into account the received signal levels of the serving Node B and neighbor Node Bs around the time of step 211 in step 240, it transmits a handover request message to the serving Node B or the target Node B. In the present invention, it is assumed that the UE transmits the handover request message to the serving Node B in step 213. Then the serving Node B transmits the handover request message to the target Node B in step 215.
For the NACK message received from the UE in step 211, the serving Node B retransmits PDU #k+1 and the resource information to the UE in step 219. If the UE fails to receive PDU #k+1 or the resource information, it transmits a NACK message to the serving Node B, indicating the reception failure of PDU #k+1 in step 221.
Meanwhile, the target Node B negotiates about the handover with the serving Node B and the RNC in step 217. During the negotiation, the target Node B decides as to whether to accept the handover of the UE. If determining to accept the handover, the target Node B requests the serving Node B to transmit the sequence numbers of PDUs that the UE has successfully received so far and the sequence numbers of buffered PDUs to be transmitted to the UE by a Radio Link Protocol (RLP) state request message in step 227.
Upon receipt of the NACK message in step 221, the serving Node B retransmits PDU #k+1 and the resource information to the UE, despite reception of the RLP state request message in step 223.
While not shown, the serving Node B can obviously transmit a new PDU to the UE before receiving the RLP state request message from the target Node B.
When receiving the RLP state request message, the serving Node B retransmits PDU #k+1 up to a maximum number of transmission times and transmits a response for the RLP state request message.
In the illustrated case of FIG. 2, the UE has successfully received PDU #k+1 and the resource information before the maximum number of transmission times is reached. Therefore, the UE transmits an ACK message to the serving Node B, indicating the successful reception of PDU #k+1 or the resource information in step 225.
Upon receipt of the ACK message, the serving Node B transmits the sequence numbers of the PDUs transmitted to the UE so far and the sequence numbers of the buffered PDUs for the UE to the target Node B by an RLP state response message in step 229.
As described above, when the UE is located in the handover zone, it is placed in a poor channel status or affected by neighbor cell interference. Therefore, data retransmission occurs a plurality of times until successful reception of data in the UE. The handover negotiation is not completed until the serving Node B receives the ACK message for PDU #k+1. Hence, the handover procedure takes a long time.
While not shown in FIG. 2, if the UE fails to decode data even though the serving Node B retransmits the data up to the maximum number of transmission times, the UE transmits a NACK message to the BS after the last retransmission. Subsequently, the serving Node B transmits the sequence numbers of the PDUs transmitted to the UE so far and the sequence numbers of the buffered PDUs for the UE to the target Node B by an RLP state response message. Clearly, the buffered PDUs include PDU #k+1.
As a consequence, the handover procedure takes much time, and additional transmission of PDU #k+1 from the RNC to the target Node B and additional transmission of PDU #k+1 from the target Node B to the UE involving coding and modulation further increase the handover time and a transmission delay considerably.
After receiving the RLP state response message, the target Node B receives PDU #k+2 from the RNC in step 231 and transmits PDU #k+2 and resource information to the UE in step 233. Then the UE transmits an ACK message for PDU #k+2 to the target Node B in step 235.
As described above, when the UE is in the handover zone, there is a high probability of data loss during data transmission from the serving Node B to the UE. Thus, the serving Node B should retransmit the lost data to the UE. However, there is no specified method for preventing unnecessary data retransmission.